Object detector, sensing device, and mobile apparatus

ABSTRACT

An object detector including a light-emitting system to emit light to an object, a light detector, a signal detector, and a threshold adjuster. The light detector receives the light emitted from the light-emitting system and reflected by the object, and output a signal. The signal detector detects the signal output from the light detector based on a threshold value of voltage. The threshold adjuster changes the threshold value between when the light-emitting system emits light to a part of a light-emission range of the light-emitting system and when the light-emitting system emits light to other part of the light-emission range other than the part of the light-emission range.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. application Ser. No.15/616,359, Jun. 7, 2017, which is based on and claims priority pursuantto 35 U.S.C. § 119(a) to Japanese Patent Application No. 2016-115957,filed on Jun. 10, 2016 in the Japan Patent Office, the entiredisclosures of each are hereby incorporated by reference herein.

BACKGROUND Technical Field

Embodiment of the present disclosure relate to an object detector, asensing device, and a mobile apparatus.

Related Art

An apparatus has been known that emits light to a light-emission rangeand receives the light reflected and scattered by an object to detectinformation regarding the object, such as the presence of the object andthe distance to the object.

However, such an apparatus has room for improvement in preventing areduction in detection distance within the projection range.

SUMMARY

In one aspect of this disclosure, there is provided an improved objectdetector including a light-emitting system to emit light to an object, alight detector, a signal detector, and a threshold adjuster. The lightdetector receives the light emitted from the light-emitting system andreflected by the object, and output a signal. The signal detectordetects the signal output from the light detector based on a thresholdvalue of voltage. The threshold adjuster changes the threshold valuebetween when the light-emitting system emits light to a part of alight-emission range of the light-emitting system and when thelight-emitting system emits light to other part of the light-emissionrange other than the part of the light-emission range.

In another aspect of this disclosure, there is provided an improvedobject detector including a light-emitting system, a light detector, anda signal detector. The light detector includes a light-receiving elementto receive light emitted from the light-emitting system and reflected bythe object, and output a signal. The signal detector detects the signaloutput from the light detector based on a threshold value of voltage. Adegree of sensitivity of the light detector differs between when thelight-emitting system emits light to a part of a light-emission range ofthe light-emitting system and when the light-emitting system emits lightto another part of the light-emission range.

In still another aspect of this disclosure, there is provided animproved sensing device including the above-described object detectorand a monitoring controller. The monitoring controller determines atleast one of a presence or an absence of the object, a direction ofmovement of the object, and a moving speed of the object, based on anoutput of the object detector.

In yet another aspect of this disclosure, there is provided an improvedmobile apparatus including a mobile object and the above-describedobject detector mounted on the mobile object.

In further aspect of this disclosure, there is provided an improvedmobile object including a mobile object and the above-described sensingdevice mounted on the mobile object.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned and other aspects, features, and advantages of thepresent disclosure will be better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings, wherein:

FIG. 1 is a block diagram of a schematic configuration of an objectdetector according to an embodiment of the present disclosure;

FIG. 2A is an illustration of a projection optical system and asynchronous system;

FIG. 2B is an illustration of a light-receiving optical system;

FIG. 2C is a schematic view of an optical path from a laser diode (LD)to a reflection mirror and another optical path from the reflectionmirror to a time-measuring photo diode (PD);

FIG. 3A is a schematic illustration of configuration of a first opticaldetector according to an embodiment of the present disclosure;

FIG. 3B is a schematic illustration of configuration of a second opticaldetector according to an embodiment of the present disclosure;

FIGS. 4A through 4C each is an illustration of an example in which adetection range is divided into a plurality of detection areas;

FIGS. 5A through 5C are waveform charts of received-light signals outputfrom light detectors, respectively;

FIGS. 6A and 6B are graphs for describing an example of a method fordividing a plurality of detection areas of a detection range into aplurality of groups;

FIG. 7 is an illustration for describing an example of the plurality ofgroups of the plurality of detection areas of the detection range;

FIG. 8A is an illustration of the amount of received light in emittinglight to the center of the detection range;

FIG. 8B is an illustration of the amount of received light in emittinglight to any edge of the detection range;

FIG. 9 is an illustration for describing a method for setting athreshold voltage using a voltage adjuster;

FIGS. 10A-1, 10A-2, and 10A-3 are waveform charts of received-lightsignals from the light detectors, respectively each light detectorhaving the same detection sensitivity over the entire detection areas;

FIGS. 10B-1, 10B-2, and 10B-3 are waveform charts of received-lightsignals from the light detectors, respectively each light detectorhaving a different detection sensitivity for each detection area;

FIGS. 11A and 11B are graphs for describing another example of a methodfor dividing a plurality of detection areas of a detection range into aplurality of groups;

FIG. 12 is an illustration for describing another example of theplurality of groups of the plurality of detection areas of the detectionrange;

FIG. 13 is an illustration of a sensing device;

FIG. 14 is a block diagram of a configuration of an object detectoraccording to a variation of the present disclosure;

FIG. 15A is an illustration of a waveform of a received-light signalwith a great shot noise; and

FIG. 15B is an illustration of a waveform of a received-light signalwith a small shot noise.

The accompanying drawings are intended to depict embodiments of thepresent disclosure and should not be interpreted to limit the scopethereof. The accompanying drawings are not to be considered as drawn toscale unless explicitly noted.

DETAILED DESCRIPTION

In describing embodiments illustrated in the drawings, specificterminology is employed for the sake of clarity. However, the disclosureof this patent specification is not intended to be limited to thespecific terminology so selected and it is to be understood that eachspecific element includes all technical equivalents that have the samefunction, operate in a similar manner, and achieve similar results.

Although the embodiments are described with technical limitations withreference to the attached drawings, such description is not intended tolimit the scope of the disclosure and all of the components or elementsdescribed in the embodiments of this disclosure are not necessarilyindispensable.

In recent years, a measuring device for measuring a round-trip distanceto a target using the TOF method has been widely used in the industriesof the technology for sensing vehicles, the motion-capture technology,and the range instruments. Such a measuring device includes alight-emitting device, a light-receiving element, and various drivecircuits. The measuring device causes the light-emitting element to emita light beam to a measurement target and causes the light-receivingelement to receive light reflected by the measurement target. Morespecifically, the measurement device detects a difference between thelight-emitting timing and the light-receiving timing as well as thephase delay through signal processing of a signal processor, thusmeasuring a round-trip distance to the target.

As an example of the measurement device, LiDARs are widely used to bemounted in air planes, railways, and vehicles. There are various LiDARssuch as a scanning LiDAR. In a scanning LiDAR as an example, a lightsource emits a laser beam, and a rotating mirror scans a scanning rangewith the emitted laser beam. Then, a light detector detects the lightreflected or scattered from an object using the rotating mirror, thusdetecting the presence of the object within a desired range and thedistance to the object.

Such a scanning LiDAR, which scans both an area illuminated by a laserbeam and an area to be detected by a detector, concentrates a laser beamon only a portion to be detected, thereby minimizing the detectablerange of the detector. Thus, the scanning LiDAR is advantageous for theaccuracy of detection and the detection distance, and the cost for thedetecto.

However, the scanning LiDAR fails to accumulate data, and therebydetection errors are more likely to occur particularly in the detectionof an object at long range than the non-scanning LiDAR does. That is,the scanning LiDAR has difficulties in increasing the accuracy ofdetection in long-range finding.

To handle such circumstances, there is a need for the detectabledistance to be of the order of 100 m. In general, the amount of lightreflected from the object in a distance of 100 m is approximately fromseveral nW through several dozen nW. In other words, a light-receivingsystem preferably detects received-light signals without any detectionerrors. The received-light signal relative to weak light ofapproximately several nW has a small signal strength so that thereceived-light signal is vulnerable to random noise, which adverselyaffects the accuracy of distance measurement and the reliability ofobject detection.

The above-described random noise includes circuit noise and shot noise.The shot noise particularly has an adverse effect. The circuit noise,which is approximately several millivolts (mV) in general, is created bythermal noise due to resistance or radiation noise picked up by thesubstrate.

In contrast, the shot noise is white noise generated in light intensitymeasurement. The degree of shot noise is proportional to the square rootof time-averaged amount of light. The shot noise might increase to beseveral tens mV or more when sensitivity or disturbance light isintense. This is why the shot noise is more likely to have adverseeffects on the accuracy of distance measurement and the reliability ofobject detection. The shot noise is generated in DC (direct current)light detection as well, which is clear from the fact that the degree ofshot noise is proportional to the square root of the time-averagedamount of light.

In a system that detects a received-light signal based on a thresholdvoltage, the threshold voltage is sufficiently increased relative toshot noise to prevent an erroneous detection due to noise. Accordingly,the threshold voltage is determined assuming that shot noise is maximum(see FIG. 15A). However, such a determined threshold voltage isexcessively large for relatively small shot noise (see FIG. 15B), whichreduces the detection distance to an excessive degree. Thus, thethreshold voltage is preferably set as small as possible within therange in which no erroneous detection occurs, so that the detectiondistance can be increased.

To handle such circumstances, the presence or absence of noise may bedetermined based on frequency characteristics of a received signal whenthe received signal exceeds a threshold value before the emission of alaser beam. However, such a method employs an analog-to-digital (AD) andan amplitude detector, which leads to an increase in size and cost of acircuit.

There is another method for adjusting an amplification factor of areceived-light signal to a maximum level within the rage that anyerroneous detection due to noise does not occurs, during the pause of alight-emitting device for a certain period. However, such a method,which involves a pause period, might fail to set the amplificationfactor to a specified value when a frame rate is limited or the noiselevel gently changes over time (for example, the intensity of extraneouslight gently changes). The method also employs a variable gainamplifier, thus increasing the cost.

In view of the above circumstances, the present inventor has conceivedof the following embodiments to achieve an object detector that enablesa long distance detection even in areas having a low light utilizationefficiency in effective manners without an increase in size and cost ofa circuit and without a use of a feedback circuit.

A description is provided of an object detector 100 according to oneembodiment of the present disclosure referring to the drawings.

FIG. 1 is a block diagram of a schematic configuration of the objectdetector 100.

The object detector 100 is a LiDAR (Light Detection and Ranging) deviceto detect the presence of an object, such as a preceding vehicle, aparked vehicle, a structure, or a pedestrian, and the distance to theobject. The object detector 100, which is mounted on a vehicle, e.g., anautomobile, as a movable body, is powered from a vehicle battery. In thepresent embodiment, a scanning LiDAR is used as the object detector 100.In some embodiments, a non-scanning LiDAR may be used instead.

As illustrated in FIG. 1, the object detector 100 includes alight-emitting system 10, a light receiving optical system 30, adetection system 40, a time measuring device 45, a synchronous system50, a measurement controller 46, and an object recognizer 47.

The light-emitting system 10 includes a laser diode LD10 as a lightsource, an LD drive device 12, and a projection optical system 20.

The laser diode L10, which is also called an end-surface emitting laser,is driven by the LD drive device 12 (a drive circuit) to emit a laserbeam. The LD drive device 12 causes the laser diode LD10 to emit a laserbeam, using a LD drive signal, which is a rectangular pulse signal,output from the measurement controller 46. Examples of the LD drivedevice 12 include a capacitor connected to the laser diode LD10 tosupply electric current to the laser diode LD10, a transistor to switchconduction and non-conduction between the capacitor and the laser diodeLD10, and a charger to charge the capacitor. The measurement controller46 starts or stops the measurement in response to a measurement controlsignal (a measurement start signal and a measurement stop signal) froman electronic control unit (ECU) on vehicle.

FIG. 2A schematically illustrates the projection optical system 20 andthe synchronous system 50. FIG. 2B schematically illustrates the lightreceiving optical system 30. A description is provided below of theprojection optical system 20, the synchronous system 50, and the lightreceiving optical system 30, using an XYZ three-dimensional rectangularcoordinate system illustrated in FIGS. 2A, 2B, and 2C as appropriate, inwhich a vertical direction is a Z-axis direction.

The projection optical system 20 includes a coupling lens 22, areflection mirror 24, and a rotating mirror 26 as a deflector. Thecoupling lens 22 is disposed on the optical path of light emitted fromthe laser diode LD10. The reflection mirror 24 is disposed on theoptical path of the light having passed through the coupling lens 22.The rotating mirror 26 is disposed on the optical path of the lightreflected from the reflection mirror 24. In this case, the reflectionmirror 24 is disposed on the optical path between the coupling lens 22and the rotating mirror 26, such that the optical path is folded toreduce the size of the detector 100.

In the optical path, the light emitted from the laser diode LD10 passesthrough the coupling lens 22 to be shaped into a predetermined beamprofile, and the shaped light is then reflected by the reflection mirror24. The rotating mirror 26 deflects the reflected light around the axisZ within a predetermined range of deflection.

The light deflected by the rotating mirror 26 within the predeterminedrange of deflection corresponds to light projected by the projectionoptical system 20, that is, light projected from the object detector100.

The rotating mirror 26 includes a plurality of reflection planes aroundthe axis of rotation (axis Z) to reflect (deflect) the light reflectedfrom the reflection mirror 24 while rotating around the axis ofrotation, thereby causing the light to unidimensionally scan aneffective scan area corresponding to the range of deflection in ahorizontal one-axis direction (Y-axis direction). In this case, therange of deflection, i.e., the effective scan area lies on +X side.Hereinafter, the direction of rotation of the rotating mirror 26 isreferred to as a “direction of rotation of mirror”. In the presentdisclosure, the effective scan area is referred to also as a projectionrange or a detection range.

As illustrated in FIG. 2A, the rotating mirror 26 includes tworeflection planes opposed to each other. However, the present disclosureis not limited to the configuration. In some embodiments, the rotatingmirror 26 may include one reflection plane or three or more reflectionplanes. Alternatively, in some embodiments, the rotating mirror 26includes at least two reflection planes, which are tilted at differentangles with respect to the axis of rotation (axis Z), to switch an areato be scanned and detected in Z-axis direction.

The light receiving optical system 30 includes, as illustrated in FIG.2B, the rotating mirror 26, the reflection mirror 24, and animage-forming optical system. The rotating mirror 26 reflects lightprojected from the projection optical system 20 and reflected(scattered) by an object disposed within an effective scan area. Thereflection mirror 24 reflects the light reflected from the rotatingmirror 26. The image-forming optical system forms an image of the lightreflected from the reflection mirror 24 onto a time measuring PD 42.

FIG. 2C is an illustration of an optical path between the laser diodeLD10 and the reflection mirror 24 and another optical path between thereflection mirror 24 and the time measuring PD 42.

As is clear from FIG. 2C, the projection optical system 20 and the lightreceiving optical system 30 overlap in Z-axis direction. The rotatingmirror 26 and the reflection mirror 24 are common between the projectionoptical system 20 and the light receiving optical system 30. Such aconfiguration reduces relative misalignment between the range ofillumination of the LD and the range of light reception of the timemeasuring PD 42 on an object, thus achieving stable detection of theobject.

The light projected from the projection optical system 20 and reflected(scattered) by an object proceeds, via the rotating mirror 26 and thereflection mirror 24, through the image-forming optical system, therebycollecting on the time measuring PD 42, referring to FIG. 2B. In FIG.2B, the reflection mirror 24 is disposed between the rotating mirror 26and the image-forming optical system, folding the optical path to reducethe size of the system. In this case, the image-forming optical systemincludes two lenses (image-forming lenses). However, in someembodiments, the image-forming optical system may include a single lensor three or more lenses. Alternatively, in some embodiments, a mirroroptical system may be employed for the image-forming optical system.

Referring to FIG. 1, the detection system 40 includes a first lightdetector 43 (sometimes referred to simply as a light detector) and afirst binarizing circuit 44 (comparator).

FIG. 3A and FIG. 3B are an illustration of a light detector 43-1 andanother light detector 43-2, respectively as example configurations ofthe first light detector 43.

As illustrated in FIGS. 3A and 3B, the first light detector 43 includesthe time measuring PD 42 (photodiode) as a light-receiving element and aprocessing circuit 60 (60-1 or 60-2). The time measuring PD 42 receiveslight that has been emitted from the projection optical system 20,reflected or scattered by an object within the effective scan area, andhas passed through the light receiving optical system 30.

Referring to FIGS. 3A and 3B, each of the processing circuits 60-1 and60-2 of the first light detector 43 includes a current-voltage converter60 a, such as a Transimpedance Amplifier (TIA), and a signal amplifier60 b such as a high-linearity analog variable-gain amplifier (VGA). Thecurrent-voltage converter 60 a converts the output current (currentvalue) from the time measuring PD 42 into a voltage signal (voltagevalue). The signal amplifier 60 b amplifies the voltage signal outputfrom the current-voltage converter 60 a. The processing circuit 60-2includes a high-pass filter (HPF) 60 c in the post-stage of the signalamplifier 60 b.

The first binarizing circuit 44 binarizes an analog voltage signaloutput from the processing circuit 60 of the first light detector 43based on a threshold voltage value and outputs the binarized signal(digital signal) as a detected signal to the time measuring device 45.

Referring to FIG. 1, the synchronous system 50 includes a second lightdetector 53 and a second binarizing circuit 56.

As illustrated in FIGS. 1 and 2A, the second light detector 53 includesa synchronization lens 52, a synchronization detection PD 54 as anotherlight-receiving element, and another processing circuit. Thesynchronization lens 52 is disposed in an optical path of light that isemitted from the laser diode LD10 and passes through the coupling lens22 to be reflected by the reflection mirror 24 and deflected by therotating mirror 26, coming back to the reflection mirror 24 to bereflected thereby again. The synchronization detection PD 54 is disposedon the optical path of the light having passed through thesynchronization lens 52. The described-above another processing circuitprocesses electric current output from the synchronization detection PD54. The described-above another processing circuit of the second lightdetector 53 has the same configuration as that of the processing circuit60-1 or the processing circuit 60-2 of the first light detector 43.

Specifically, the reflection mirror 24 is disposed upstream from therange of deflection in the direction of rotation of the rotating mirror26. The light deflected by the rotating mirror 26 toward upstream fromthe range of deflection enters the reflection mirror 24. The lightdeflected by the rotating mirror 26 and reflected by the reflectionmirror 24 passes through the synchronization lens 52 and enters thesynchronization detection PD 54. Then, the synchronization detection PD54 outputs the electric current to another processing circuit in thesecond light detector 53.

Note that, in some embodiments, the reflection mirror 24 may be disposeddownstream from the range of deflection in the direction of rotation ofthe rotating mirror 26. Further, the synchronous system 50 may bedisposed in the optical path of the light deflected by the rotatingmirror 26 and reflected by the reflection mirror 24.

The rotating mirror 26 rotates to guide the light reflected by thereflection plane of the rotating mirror 26 to the reflection mirror 24,and the light reflected by the reflection mirror 24 enters thesynchronization detection PD 54. The synchronization detection PD 54having received the light outputs electric current, which occurs uponeach receipt of light. That is, the synchronization detection PD 54periodically outputs electric current to the second PD output detector56.

The light emission for synchronization described above, which irradiatesthe synchronization detection PD 54 with light deflected by the rotatingmirror 26, allows obtaining the timing of rotation of the rotatingmirror 26 based on the timing at which the synchronization detection PD54 receives light.

With a predetermined length of time elapsed after the laser diode LD10has emitted light for synchronization, the LD10 emitting pulsed lightallows the effective scan area to be optically scanned with the emittedpulsed light. That is, the laser diode LD10 emits pulsed light during aperiod before and after the timing at which the synchronizationdetection PD 54 receives light, thereby optically scanning the effectivescan area.

In this case, examples of the light-receiving element for measuring timeand detecting synchronization include a photo diode (PD) as describedabove, an avalanche photo diode (APD), and a single photon avalanchediode (SPAD) as a Geiger mode APD. The APD and the SPAD have highersensitivity than a PD, and thus is advantageous in the accuracy ofdetection or the detectable distance.

The second binarizing circuit 56 binarizes an analog voltage signaloutput from the processing circuit of the second light detector 53 basedon a threshold voltage value and outputs the binarized signal (digitalsignal) as a detected signal to the measurement controller 46.

The measurement controller 46 generates an LD drive signal based on thesynchronization signal from the second binarizing circuit 56, andoutputs the LD drive signal to the LD drive device 12 and the timemeasuring device 45.

That is, the LD drive signal is a light-emission control signal(periodic pulsed signal) which is delayed relative to thesynchronization signal.

When receiving the LD drive signal, the LD drive device 12 applies adrive current to the laser diode LD10. The laser diode LD10 then outputspulsed light. In this case, the duty of the pulsed light emitted fromthe laser diode LD10 is restricted in consideration for the safety anddurability of the laser diode LD10. This is why the pulse width of thepulsed light emitted from the laser diode LD10 is preferably narrower.The pulse width is generally set in a range from approximately 10 nsthrough approximately several dozen ns. The pulse interval isapproximately several dozen microseconds in general.

The time measuring device 45 calculates a difference in input timingbetween the input timing of the LD drive signal output power from themeasurement controller 46 and the input timing of the detected signal(binarized signal) output from the first binarizing circuit 44, as atime difference between the timing of light emission of the laser diodesLD10 and the timing of light reception of the time measuring PD 42,outputting the calculated time difference (a time measurement result) tothe measurement controller 46.

The measurement controller 46 converts the measurement result from thetime measuring device 45 into distance to obtain a round-trip distanceto and from an object, and outputs one-half of the round-trip distanceas distance data to the object recognizer 47.

The object recognizer 47 recognizes the position of an object based on aplurality of sets of distance data obtained by one or more scans,outputting an object recognition result to the measurement controller46. The measurement controller 46 transfers the object recognitionresult to the ECU.

The ECU performs, based on the transferred object recognizer, steeringcontrol of a vehicle, such as auto-steering, and speed control, such asauto-braking.

Hereinafter, a description is also given of a non-scanning objectdetector in addition to a scanning object detector. Note that, thenon-scanning object detector projects light emitted from, for example, alight source, in a direct manner or through a lens.

FIG. 4A through FIG. 4C each is an illustration of an example in whichthe detection range of the object detector 100 is divided into detectionareas. In FIG. 4A, the detection range of the object detector 100 has asubstantially fan shape as a whole when viewed from the Z-axisdirection. In FIG. 4B, the detection range of the object detector 100has a substantially rectangular shape as a whole when viewed from theZ-axis direction. Hereinafter, the detection range in a substantiallyfan shape is referred to also as a detection angle range.

As illustrated in FIGS. 4A and 4B, the detection range is horizontallydivided into detection areas. This configuration can be achieved bydividing the detection range at least in receiving light. Morespecifically, for example, at least one light-emitting element emitsdiverging light and a plurality of light detectors receive lightreflected from an object, thereby obtaining such a divided detectionrange. In FIG. 4A, the detection range is horizontally divided intothree detection areas D1, D2, and D3.

The detection range horizontally divided into detection areas (D1through D4) as illustrated in FIG. 4B is obtained by dividing thedetection range at least in emitting light. More specifically, forexample, a plurality of light-emitting elements emit light and at leastone light detector receives light reflected from an object, therebyobtaining the divided detection range. In FIG. 4B, the detection rangeis horizontally divided into four detection areas D1, D2, D3, and D4.

For another example, a detection range may be vertically divided intodetection areas as illustrated in FIG. 4C. Such a detection range canalso be obtained by dividing the detection range at least in emitting orreceiving light. In FIG. 4C, the detection range is vertical directiondivided into four detection areas D1, D2, D3, and D4.

The detection range may be divided into detection areastwo-dimensionally, i.e., horizontally or vertically. In FIGS. 4A through4C, the detection range is divided into a small number of detectionareas, such as three to six, for the sake of simplification.Alternatively, cases in which the detection range is divided intoseveral tens to several thousands of detection areas are also within thepresent disclosure.

In a system that detects a received-light signal (an output signal of alight detector) based on a threshold voltage, the threshold voltage issufficiently increased relative to noise to prevent an erroneousdetection due to noise. However, excessively increasing the thresholdvoltage shortens the detection distance to an excessive degree.

The intensity distribution of shot noise conforms to the normaldistribution with an average of 0. Accordingly, when the upper limit ofthe probability (probability of erroneous detection) that the degree ofnoise exceeds the threshold value is determined as the specification ofthe system, the detection distance can be maximum (threshold voltagevalue can be minimum) within the range that the probability of erroneousdetection fall within the specification of the system. Thus, the ratioof ideal threshold voltage relative to the standard deviation of noisecan be determined.

That is, when the standard deviation of noise is obtained, idealthreshold voltage is determined accordingly. For example, when theprobability of erroneous detection is reduced to be 0.2% or less, thethreshold voltage is greater than or equal to any value obtained by theexpression “3×standard deviation of noise” (threshold value≥3×standarddeviation of noise) according to the probability density function of thenormal distribution. When the threshold voltage is equal to the value ofthe expression “3×standard deviation of noise”, a maximum detectiondistance can be obtained while satisfying the range of theabove-described expression. For example, when the standard deviation ofnoise is 10 millivolts (mV), the threshold voltage is preferably 30 mV.

Hence, the ratio of the threshold voltage relative to the standarddeviation of noise preferably remains the same to obtain a maximumdetection distance without increasing the erroneous detectionprobability.

The ratio of the threshold voltage relative to ideal standard deviationof noise is preferably within the range of from 3 to 10 to prevent anerroneous detection and achieve a long-range detection although such aratio depends on a permissible erroneous detection probability.

First Example

The following describes a first example in which a single light-emittingelement emits a diverging light beam to an object and three lightdetectors receive the light beam reflected from the object, dividing adetection range into three detection areas, as illustrated in FIG. 4A.

In the present example, light detectors LD1, LD2, and LD3 correspond todetection areas D1, D2, and D3, respectively. The light receivingoptical system 30 has light utilization efficiency η1, η2, and η3relative to the light detectors LD1, LD2, and LD3. The time measuring PD42 of the first light detector 43 detects received-light signals RS1,RS2, and RS3 of the light detectors LD1, LD2, and LD3, based on thethreshold voltages Vth1, Vth2, and Vth3, respectively.

In the light receiving optical system 30, the constituent elements of anoptical member to guide a light beam to each light detector may differbetween the light detectors LD1, LD2, and LD3 or may be common among thelight detectors LD1, LD2, and LD3.

The amounts of disturbance light that enter light detector LD1, LD2, andLD3 are proportional to the light utilization efficiency η1, η2, and η3,respectively. The degree of shot noise is proportional to the squareroot of the amount of disturbance light. Accordingly, σ1 is proportionalto the square root of η1, σ2 is proportional to the square root of η2,σ3 is proportional to the square root of η3 where σ1, σ2, and σ3represent the degrees of noise of received-light signals RS1, RS2, andRS3 of the light detectors LD1, LD2, and LD3, respectively.

The following relation of changes in threshold voltage with noise ismost ideal to equalize the ratio of the threshold voltage relative tothe standard deviation of noise between the three detection areas D1,D2, and D3: Vth 2=V0, Vth1=V0×√(η1/η2), and Vth3=V0×√(η3/η2). In thiscase, V0 is determined to obtain an ideal value for the ratio of thethreshold voltage relative to the standard deviation of noise in thedetection area D2, as illustrated in FIGS. 5A through 5C.

Note that, the standard deviation of noise is not measured in real timebut a preliminarily assumed value for the standard deviation of noise isdetermined through experiments or calculation.

That is, the process that measures noise and feeds back to a thresholdvalue is not performed in the present embodiment. Instead, any thresholdvalue for each detection area is determined in design.

Despite the above-described most ideal case as illustrated in FIGS. 5Athrough 5C, the advantageous effects (to prevent an erroneous detectionand achieve a long-range detection) can be achieved by satisfying therelation that reduces the threshold voltage in a detection area with alow degree of noise as compared to another detection area with a highdegree of noise. That is, the advantageous effects can be obtained withmore uneven parameters without the parameters as illustrated in FIGS. 5Athrough 5C.

When a direct current (DC) component of disturbance light issuperimposed on a received-light signal, accurately detecting pulse(pulsed light) is difficult. For this reason, a high-pass filter (HPF)60 c is disposed in a circuit for generating a received-light signal asillustrated in FIG. 3B. In FIGS. 5A through 5C, the DC components arecut off.

In the present embodiment described above, the threshold voltage isdetermined using the light utilization efficiency of the light receivingoptical system 30. Alternatively, in some embodiments, the thresholdvalue may be determined based on an experimental comparison of thedegrees of noise in the respective detection areas having received lightwith a constant intensity output from the object detector 100.

In a method that detects a plurality of areas within a detection rangeby scanning the detection range with a laser beam, high angularresolution in the scanning direction can be achieved. Thus, the numberof divisions of the detection range might range from several hundred toseveral thousand. As a method for determining different threshold valuesfor the detection areas, the threshold value may be changed withdiscrete values, for example by determining a different threshold valuefor each detection area. In this case, a threshold voltage may be setfor each detection area. This method, however, fails to exhibit anyadvantageous effects because finely adjusting the threshold value ismeaningless in a practical use and such an adjustment leads to anincrease in size of the processing circuit 60.

This is because, the difference between the detection area (the former)with the largest light utilization efficiency and the detection area(the latter) with the smallest light utilization efficiency is at mostapproximately ten times as large as the light utilization efficiency ofthe latter. Accordingly, the threshold value of the latter differs fromthe threshold value of the former by approximately √10 times of thethreshold value of the former. When the difference is divided by severalhundred, the divided value is less than or equal to 1% of the maximumthreshold value, which is too small to be noticed among variations incircuit constant.

In view of the above, it is practical that a plurality of detectionareas (a detection range) are divided into fewer groups (for example,seven groups in the present embodiment) and a threshold voltage isdetermined for each group. The following describes a method for dividingthe detection range into groups and a method for determining a thresholdvoltage for each group.

FIG. 6A represents a graph of a change in square root of lightutilization efficiency η of the light receiving optical system 30, withdetection angle where the detection angle ranges from 0° to 120°. FIG.6B represents a graph of threshold voltages for the detection angles forgroups, respectively. In FIGS. 6A and 6B, broken lines are spacedevenly.

The threshold voltage values Vth1, Vth2, Vth7 correspond to group 1,group 2, . . . group 7.

As illustrated in FIG. 6B, a plurality of detection areas are dividedinto groups with a detection angle partitioning two adjacent groupswhere the threshold voltage changes with the detection angle. Thisconfiguration enables the detection areas to be effectively divided intogroups, thereby favorably determining the threshold voltage.

FIG. 7 is an illustration of the plurality of detection areas, which areillustrated in FIG. 6, divided into groups. In FIG. 6, the thresholdvoltage is determined based on the square root of light utilizationefficiency. Alternatively, in some embodiments, the threshold voltagemay be determined based on the degree of noise for each detection anglewhen the object detector 100 emits light with a constant intensity.

In the present embodiment, the plurality of detection areas are dividedinto seven groups. Alternatively, in some embodiments, the plurality ofdetection areas may be divided into fewer or more groups than seven aslong as the number of groups is less than the number of detection areas.Further, in the present embodiment, the variation width of the thresholdvoltage is constant, but no limitation is intended hereby. In someembodiments, the variation width of the threshold voltage may not beconstant.

Alternatively, in some embodiments, the threshold voltage may becontinuously changed in an analog manner. In this case, the detectionrange may be divided into fewer groups than the number of detectionareas to determine different threshold voltage values. Alternatively,the threshold voltage may be changed with light utilization efficiencyfor each detection area of the light receiving optical system 30.

When a plurality of light-receiving elements, a plurality oflight-emitting elements, or the combination thereof are disposed in theobject detector 100, a plurality of detection areas are detected asillustrated in FIGS. 4A and 4B. In this case, setting substantially thesame light utilization efficiency of the light receiving optical systemfor each detection area unsuccessfully increases the size of the objectdetector 100 due to an increase in effective diameter of an opticalelement, such as a lens or a mirror.

To avoid such circumstances, the light utilization efficiency at an edgeof the detection range is reduced as compared to the light utilizationefficiency at the center of the detection range, thereby reducing theoptical element in size. Further, the threshold value at any edge of thedetection range is reduced as compared to the threshold value at thecenter of the detection range, thereby preventing an increase inerroneous detection probability while preventing a reduction indetection distance at any edge of the detection range due to thedownsize of the optical element.

The configuration that emits a laser beam to a scanning device, such asthe rotating mirror 26, from the outside of the detection angle range isdisadvantageous to increase the detection distance at the edge of thedetection range. However, the configuration is advantageous formulti-layer detection that employs a plurality of light-emittingelements.

Thus, such a configuration can exhibit the advantageous effects of thepresent disclosure. The following describes the reasons for the above.Note that the above-described “multi-layer detection” refers to adetection of the detection range that is divided into a plurality ofdetection areas in the Z-axis direction as illustrated in FIG. 4C.

In the configuration that emits a laser beam to the scanning device, forexample, the rotating mirror 26, from the outside of the detection anglerange, vignetting occurs in the image-forming lens during the detectionof the edges of the detection range, thereby reducing the lightutilization efficiency at any edge of the detection range as compared tothe light utilization efficiency at the center of the detection range(see FIGS. 8A and 8B).

In contrast, for example, a rotating mirror having a mirror surface thatforms an angle of 45° relative to the Z-axis is rotated around theZ-axis to perform the wide-angle detection with light parallel to theZ-axis. In such a configuration, vignetting of a light beam does notoccurs, and thus the light utilization efficiency does not depend on thescanning angle in principle.

In the configuration for the multi-layer detection in which a pluralityof of light-emitting elements are disposed along the Z-axis directionand each light-emitting element emits light to a different detectionarea arranged along the Z-axis direction, the projection beam and thedetection area are not distorted in principle when the projection beamis emitted to the scanning device from the outside of the detectionangle range.

In contrast, in the configuration that emits a light beam, which isparallel to the rotation axis, to the rotating mirror tilted at 45°relative to the rotation axis, the detection areas are distorted duringthe multi-layer detection of the plurality of light-emitting elements.In other words, for example, when any overlapping is eliminated betweenlayers of detection (i.e., between the detection areas in the Z-axisdirection) at the center of the detection range, the detection areas atany edge of the detection range are distorted, thereby causing adjacentlayers of detection (adjacent detection areas) at any edge to overlapwith each other.

For the above-described reasons, with the light receiving optical systemas illustrated in FIGS. 8A and 8B, the threshold value of an edge ofdetection rage is reduced as compared to the threshold value of thecenter of the detection range, thereby allowing for a relatively longdistance detection at the edges of the detection range as well. Further,with such a reduction, the detection areas are not distorted in themulti-layer detection, or the object detector can be downsized.

When a plurality of light-emitting elements are unidimensionallyarranged along the horizontal direction, the detection range can behorizontally divided into a plurality of detection areas as illustratedin FIGS. 4A and 4B. Further, when a plurality of light-emitting elementsare unidimensionally arranged along the vertical direction, thedetection range can be vertically divided into a plurality of detectionareas as illustrated in FIG. 4C.

Alternatively, the detection range can be two-dimensionally divided intodetection areas by arranging a plurality of light-emitting elementstwo-dimensional or unidimensional manner to emit light for scanning inboth the arrangement direction and the vertical direction. Such anarrangement is advantageous for use in a high-resolution detection witha range-finding device and to be used as a three-dimensional sensor.

When a plurality of light-emitting elements are used to divide thedetection range in emitting light and one light-receiving element isused to receive the emitted light, the plurality of light-emittingelements are configured to emit light at different timings. With such aconfiguration, only one received-light signal at one time enters thecircuit board with the light-receiving element and the processingcircuit. Accordingly, the received-light signal does not includecrosstalk, and thus successful object detection can be achieved.

In contrast, when a plurality of light-receiving elements are used todivide the detection range in receiving light and one light-emittingelement is used to emit light, the plurality of light-receiving elementsreceive light at the same time. With such a configuration,received-light signals are transmitted through the circuit board at thesame time, and thereby crosstalk might generate between thereceived-light signals.

When the detection range is divided by a plurality of light-emittingelements in emitting light and by a plurality of light-receivingelements in receiving light, fewer light-receiving element are used ascompared to the case in which the detection range is divided only by aplurality of light-receiving elements in receiving light (assuming thatthe detection range is divided to the same number of detection areasbetween both cases). Thus, crosstalk between received-light signalsdecreases, which is advantageous for object detection.

FIG. 9 is an illustration of a voltage adjuster 200 as a thresholdadjuster to set a threshold voltage for each detection area or for eachgroup in the object detector 100. The voltage adjuster 200 may beimplemented by, for example, a circuit.

The voltage adjuster 200 includes a pulse-signal generating circuit 200a, a smoothing unit 200 b, a duty controller 200 c, and a memory 200 d.The pulse-signal generating circuit 200 a generates a pulse signalincluding a plurality of pulses, each pulse variable in a duty ratio.The smoothing unit 200 b smooths the generated pulse signal. The dutycontroller 200 c controls the duty ratio of the pulse signal. The memory200 d stores a table (hereinafter, referred to as a duty-ratio table)for duty ratios that correspond to threshold voltages preliminarily setfor detection areas or groups, respectively. In the present embodiment,the duty controller 200 c receives a synchronization signal output fromthe synchronous system 50.

The following describes a method in which the voltage adjuster 200 setsa threshold value according to a detection area or a group, referring toFIG. 9. Firstly, the duty controller 200 c detects a scanning positionof a light beam based on a synchronization signal output from thesynchronous system 50. Subsequently, the duty controller 200 c selects aduty ratio from the duty ratios in the duty-ratio table stored in thememory 200 d, based on the detected position. The duty controller 200 coutputs, to the pulse-signal generating circuit 200 a, the selected dutyratio for scanning a detection area or a group that corresponds to theselected duty ratio. The pulse-signal generating circuit 200 a havingreceived the duty ratio from the duty controller 200 c generates a pulsesignal including a plurality pulses each having the received duty ratio,and outputs the generated pulse signal to the smoothing unit 200 b. Thesmoothing unit 200 b smooths the pulse signal output from thepulse-signal generating circuit 200 a. The smoothed pulse signal is usedas a threshold voltage. As described above, the duty controller 200 c ofthe voltage adjuster 200 controls the duty ratio of a pulse signal to begenerated by the pulse-signal generating circuit 200 a, therebyobtaining different threshold values between the detection areas orgroups.

Referring back to FIG. 9, the smoothing unit 200 b outputs the smoothedpulse signal, i.e., a threshold value, to an input end (the negativeterminal) of a comparator as the first binarizing circuit 44. Thecomparator compares the threshold voltage input to the negative terminalwith a received-light signal input to the other input end (the positiveterminal), and outputs a binarized signal (a detected signal) at atiming at which the received-light signal traverses the thresholdvoltage.

The smoothing unit 200 b may be an integration circuit, such as aresistance capacitor (RC) low pass filter that includes a resistor R anda capacitor C. The pulse-signal generating circuit 200 a may be, forexample, a field-programmable gate array (FPGA) as an integrationcircuit. The FPGA is also used as a control circuit, e.g., themeasurement controller 46. Thus, an increase in size of circuit can beprevented and such a circuit is easily achieved. In some embodiments,the pulse-signal generating circuit 200 a may be any circuit other thanthe FPGA.

In the above description, the method for setting a threshold voltage isapplied in a scanning object detector. The same configuration of thevoltage adjuster 200 in FIG. 9 is applicable in a non-scanning objectdetector.

In this case, when a detection range is divided by a plurality of (thesame as the number of detection areas or groups) light-emittingelements, e.g., LDs, only in emitting light, the voltage adjuster 200generates a pulse signal having a duty ratio set for the correspondingdetection area and smooths the generated pulse signal, thus setting thethreshold voltage according to the detection area, in emitting light ofany light-emitting element for each detection area or each group.

In such a case, when fewer (at least one) light-receiving elements,e.g., PDs than the number of the light-emitting elements receive light,at least two light-emitting elements emit light at different timings. Insuch a configuration, a lighting trigger signal for each light-emittingelement is preferably input to a drive circuit for light-emittingelement as well as the duty controller 200 c.

In contrast, when the same number of light-receiving elements as thenumber of the light-emitting elements receive light, all of thelight-emitting elements can emit light at the same time. Note that, whena plurality of light-receiving elements corresponds to the samelight-emitting element, a binarizing circuit is preferably disposed foreach light-receiving element because the plurality of light-receivingelements simultaneously receives light emitted from the light-emittingelement.

Further, when the detection range is divided by a plurality oflight-receiving elements such as PDs in receiving light while a singlelight-emitting element, such as an LD, is used to emit diverging light,the voltage adjuster 200 generates a pulse signal having a duty ratioset for each detection area and smooths the generated pulse signal, thussetting a threshold voltage according to the detection area, upon thelight-emitting element emitting light. Note that, a binarizing circuitis preferably disposed for each light-receiving element because theplurality of light-receiving elements simultaneously receives lightemitted from one light-emitting element.

Second Example

The following describes a second example in which a singlelight-emitting element emits a diverging light beam to an object andthree light detectors LD1, LD2, and LD3 receive the light beam reflectedfrom the object, dividing a detection range into three detection areasD1, D2, and D3, in the same manner as in the first example (see FIG.4A).

The light detectors LD1, LD2, and LD3 have sensitivity (hereinafter,referred to as “detection sensitivity”) S1, S2, and S3, respectively.Note that the sensitivity of each light detector LD1, LD2, and LD3includes both the sensitivity of the light-receiving element and theamplification factor of a signal of the processing circuit 60.

As described above, the degree of shot noise is proportional to thesquare root of light utilization efficiency as well as to the degree ofsensitivity S1, S2, and S3 of the light detectors LD1, LD2, and LD3.Accordingly, the following relations are satisfied: σ1 is proportionalto the product of S1 and the square root of η1, σ2 is proportional tothe product of S2 and the square root of η2, and σ3 is proportional tothe product of S3 and the square root of η3 where σ1, σ2, and σ3represent the degrees of noise of received-light signals RS1, RS2, andRS3 of the light detectors LD1, LD2, and LD3, respectively.

In the present example, the following relations are determined: σ1 isproportional to half the square root of η1, σ2 is proportional to halfthe square root of η2, and σ3 is proportional to half the square root ofη3. Accordingly, the threshold voltage is set the same over the entiredetection areas so that the ratio of a threshold voltage relative to thestandard deviation of noise is constant irrespective of the detectionarea. This configuration is most advantageous from the viewpoint ofpreventing an increase in erroneous detection probability and maximizinga detection distance.

FIGS. 10A-1, 10A-2, and 10A-3 are waveform charts of received-lightsignals from the light detectors LD1, LD2, and LD3, respectively eachhaving the same detection sensitivity over the entire detection areas.FIGS. 10B-1, 10B-2, and 10B-3 are waveform charts of received-lightsignals from the light detectors LD1, LD2, and LD3, respectively eachhaving a different detection sensitivity for each detection area.

As a method for changing the sensitivity of the light detector with adetection area, a device that is capable of changing an amplificationfactor may be used as the signal amplifier 60 b of the processingcircuit 60, to control the amplification factor. Alternatively, the APDto change the level of the light-receiving sensitivity with an appliedvoltage may be used for the light-receiving element to control a voltageto be applied.

More specifically, the object detector 100 employs adetection-sensitivity adjuster (a sensitivity adjuster) to adjustdetection sensitivity for each detection area or for each group.

The detection-sensitivity adjuster includes an amplification factorcontroller, which is capable of changing an amplification factor, tocontrol an amplification factor of the signal amplifier. Alternatively,when the APD is used for the light-receiving element to change thelight-receiving sensitivity with an applied voltage, thedetection-sensitivity adjuster includes a voltage controller to controla voltage to be applied to the APD. In this case, a table is stored inthe memory 200 d, listing the amplification factors of the signalamplifier or the values of voltage to be applied to the APD, which arepreliminarily determined for each detection area or for each group.

The following describes a method in which the detection sensitivityadjuster determines the degree of detection sensitivity. Firstly, thedetection sensitivity adjuster detects a scanning position of a lightbeam based on a synchronization signal output from the synchronoussystem 50, and determines an amplification factor of the signalamplifier 60 b by selecting an amplification factor for each detectionarea or each group referring to the table at the time of scanning eachdetection area or each group. Alternatively, the detection sensitivityadjuster detects a scanning position of a light beam based on asynchronization signal output from the synchronous system 50, anddetermines a value of voltage to be applied to the APD by selecting avoltage value for each detection area or each group referring to thetable at the time of scanning each detection area or each group.Accordingly, the degree of detection sensitivity differs betweendetection areas or groups.

In the above description, the method for setting the degree of detectionsensitivity is applied in a scanning object detector 100. The sameconfiguration of the detection sensitivity adjuster is applicable in anon-scanning object detector.

In this case, when a detection range is divided by a plurality of (thesame as the number of detection areas or groups) light-emittingelements, e.g., LDs, in emitting light, the detection sensitivityadjuster determines an amplification factor of the signal amplifier 60 bby selecting an amplification factor for each detection area or eachgroup from the data listed in the table, in emitting light of anylight-emitting element for each detection area or each group.Alternatively, the detection sensitivity adjuster determines a value ofvoltage to be applied to the APD by selecting a voltage value for eachdetection area or each group from data listed in the table, in emittinglight of any light-emitting element for each detection area or eachgroup. Accordingly, the degree of detection sensitivity differs betweendetection areas or groups.

In such a case, when fewer (at least one) light-receiving elements thanthe number of the light-emitting elements receive light, at least twolight-emitting elements emit light at different timings. In such aconfiguration, a lighting trigger signal for each light-emitting elementis preferably input to a drive circuit for light-emitting element aswell as the detection sensitivity adjuster.

In contrast, when the same number of light-receiving elements as thenumber of the light-emitting elements receive light, all of thelight-emitting elements can emit light at the same time. Note that, whena plurality of light-receiving elements corresponds to the samelight-emitting element, a binarizing circuit is preferably disposed foreach light-receiving element.

Further, when the detection range is divided by a plurality oflight-receiving elements such as PDs in receiving light while a singlelight-emitting element, such as an LD, is used to emit diverging light,the detection sensitivity adjuster determines an amplification factor ofthe signal amplifier 60 b by selecting an amplification factor for eachdetection area or each group from the data listed in the table, inemitting light of the light-emitting element for each detection area oreach group. Alternatively, the detection sensitivity adjuster determinesa value of voltage to be applied to the APD by selecting a voltage valuefor each detection area or each group from data listed in the table, inemitting light of the light-emitting element for each detection area oreach group. Note that, a binarizing circuit is preferably disposed foreach light-receiving element because the plurality of light-receivingelements simultaneously receives light emitted from one light-emittingelement.

When the detection range is divided by a plurality of light-receivingelement in receiving light, the light-receiving elements may havedifferent degrees of detection sensitivity corresponding to thedetection areas, respectively. In such a case, the degree of detectionsensitivity can be changed with a detection area using only thelight-receiving elements, so that no detection sensitivity adjuster isemployed.

Despite the above-described most ideal method for determining thedetection sensitivity of each light detector, the advantageous effects(to prevent an erroneous detection and achieve a long-range detection)can be achieved by satisfying the relation that increases the detectionsensitivity of a light detector in a detection area with a small noiseas compared to another detection area with a large noise. That is, theadvantageous effects can be obtained with more uneven parameters orwithout the parameters as illustrated in FIGS. 10A-1 through 10B-3.

In a method that detects a plurality of detection areas by scanning thedetection range with a laser beam, high angular resolution in thescanning direction can be achieved. Thus, the number of divisions of thedetection range might range from several hundred to several thousand. Asa method for determining different degrees of detection sensitivity forthe detection areas, the level of the light-receiving sensitivity may bechanged with discrete values, for example by changing the amplificationfactor of the signal amplifier or the level of the sensitivity of thelight-receiving element in a step manner. In this case, the detectionsensitivity may be determined for each detection area so that thedegrees of detection sensitivity correspond to the detection areas,respectively. This method, however, fails to exhibit any advantageouseffects because finely adjusting the threshold value is meaningless in apractical use and such an adjustment leads to an increase in size of theprocessing circuit 60.

This is because, the difference between the detection area (the former)with the largest light utilization efficiency and the detection area(the latter) with the smallest light utilization efficiency is at mostapproximately ten times as large as the light utilization efficiency ofthe latter. Accordingly, the detection sensitivity of the latter differsfrom the detection sensitivity of the former by approximately √10 timesof the detection sensitivity of the former. When the difference isdivided by several hundred, the divided value is less than or equal to1% of the maximum detection sensitivity, which is too small to benoticed among variations in circuit constant.

In this case as well, it is practical that a plurality of (in thepresent example, nine or more) detection areas are divided into fewergroups (in the present example, eight groups) and the degree ofdetection sensitivity is determined for each group. The same methods fordividing the detection range into groups and for determining a degree ofdetection sensitivity for each group are applicable in the secondexample as in the first example. The following describes a method fordetermining the degree of detection sensitivity.

FIG. 11A represents a graph of a change in a reciprocal of the squareroot of light utilization efficiency η of the light receiving opticalsystem 30, with detection angle where the detection angle ranges from 0°to 120°. FIG. 11B represents a graph of the degree of detectionsensitivity set for each group (a degree of detection sensitivity foreach detection angle). In FIGS. 11A and 11B, broken lines are spacedevenly.

The detection sensitivity degrees S1, S2, . . . S8 correspond to group1, group 2, . . . group 8.

FIG. 12 represents groups 1 through 8 illustrated in FIG. 11. Asillustrated in FIGS. 11A and 11B, a plurality of detection areas aredivided into groups with a detection angle partitioning two adjacentgroups where the detection sensitivity changes with the detection angle.This configuration enables effectively dividing the detection areas intogroups as well as determining the detection sensitivity.

In FIGS. 11A and 11B, the detection sensitivity is determined based onthe reciprocal of the square root of light utilization efficiency.Alternatively, in some embodiments, the detection sensitivity may bedetermined based on the degree of noise for each detection angle whenthe object detector 100 emits light with a constant intensity.

In the present embodiment, the plurality of detection areas are dividedinto eight groups. Alternatively, in some embodiments, the plurality ofdetection areas may be divided into fewer or more groups than eight aslong as the number of groups is less than the number of detection areas.

Further, in the present embodiment, the variation width of the detectionsensitivity is constant, but no limitation is intended hereby. In someembodiments, the variation width of the detection sensitivity may not beconstant. Alternatively, in some embodiments, the threshold voltage maybe continuously changed in an analog manner. In this case, the detectionrange may be divided into fewer groups than the number of detectionareas to determine different degrees of detection sensitivity betweenthe groups. Alternatively, the degree of detection sensitivity may bechanged with light utilization efficiency for each detection area oreach group.

As described above, the light utilization efficiency at any edge of thedetection range is reduced as compared to the light utilizationefficiency at the center of the detection range, thereby reducing theoptical element in size. Further, the detection sensitivity at any edgeof the detection range is increased as compared to the threshold valueat the center of the detection range, thereby preventing an increase inerroneous detection probability while preventing a reduction indetection distance at any edge of the detection range due to thedownsize of the optical element.

This configuration is advantageous for the multi-layer detection. Anobject detector with a relatively long detection distance at any edge ofthe detection range and no distortion of the detection areas in themulti-layer detection can be achieved by combining the configuration ofthe above-described optical element with the configuration of thedetermination of detection sensitivity (the detection sensitivity atedges of the detection range is increased as compared to the center ofthe detection range).

From the first viewpoint (the threshold voltage), the above-describedobject detector 100 according to the present embodiment includes thelight-emitting system 10, the first light detector 43 (sometimes simplyreferred to as the light detector), the first binarizing circuit 44, andthe voltage adjuster 200. The first light detector 43 receives lightemitted from the light-emitting system 10 and reflected or scattered byan object. The first binarizing circuit 44 (a signal detector) receivesand detects a received-light signal (an output signal) from the firstlight detector 43 based on the threshold voltage. The voltage adjuster200 changes the threshold voltage between when the light-emitting system10 emits light to a part of a light-emission range (a detection range)of the light-emitting system 10 and when the light-emitting system 10emits light to another part of the light-emission range.

This configuration changes the threshold voltage between a plurality ofareas of the light-emission range according to, for example, lightutilization efficiency for each area. Thus, a reduction in detectiondistance due to an excessively increased threshold voltage relative tonoise included in a received-light signal can be prevented.

Thus, such a configuration can prevent a reduction in detection distancewithin the light-emission range.

For example, the threshold voltage is increased in an area with agreater light utilization efficiency due to increased disturbance lightand noise. Accordingly, a reduction in detection distance as well as anerroneous detection can be prevented. For example, the threshold voltageis reduced in an area with a lower light utilization efficiency due toreduced disturbance light and noise. Accordingly, a reduction indetection distance as well as an erroneous detection can be prevented.

Setting a threshold voltage according to an area of the light-emissionrange can be achieved through a preliminarily determined sequencecontrol, and thus such a setting operation is easily achieved withoutany feedback control, exhibiting the advantageous effects reliably.

The object detector 100 further includes the light receiving opticalsystem 30 to guide light reflected by an object to the first lightdetector 43. In the light receiving optical system 30, the lightutilization efficiency of light emitted to any edge of thelight-emission range is lower than the light utilization efficiency oflight emitted to the center of the light-emission rang. In the objectdetector 100, the voltage adjuster 200 preferably sets a thresholdvoltage to be lower in emitting light to any edge of the light-emissionrange than in emitting light to the center of the light-emission range.

This configuration can achieve an object detector that is advantageousfor a wide-angle multi-layer detection. In non-scanning object detectorsas well, reducing the light utilization efficiency at any edge of thedetection range as compared to the light utilization efficiency at thecenter of the detection range can reduce the effective diameter of theoptical element, thus downsizing the object detector. Further, reducingthe threshold value at any edge of the detection range as compared tothe threshold value at the center of the detection range can prevent areduction in detection distance due to the downsizing of the objectdetector.

The light-emitting system 10 includes a light source, e.g., the LD 111,a scanning device, e.g., the rotating mirror 26 to scan thelight-emission range with light emitted from the light source. In thiscase, a threshold voltage is preferably set according to an angle atwhich the scanning device scans the light-emission range with the light.

The light-emitting system 10 preferably employs an incident opticalsystem (a projection optical system 20) to cause the light emitted fromthe light source to enter the scanning device from the outside of thelight-emission range.

In the configuration that makes a light beam enter the scanning devicefrom the outside of the light-emission range (scanning angle range),vignetting occurs at the edges of the light-emission range (any edge ofthe scanning angle range), and thereby the light utilization efficiencysignificantly varies with the scanning angle. In such a case, theadvantageous effects of the present disclosure is significantlyexhibited. The above-described configuration is particularlyadvantageous for cases where a plurality of light-emitting elements arevertically disposed having a vertically-variable resolution.

For example, in a configuration that makes a light beam enter a rotatingmirror (having a mirror surface that forms an angle of 45° relative tothe vertical direction) from above, the light utilization efficiency isless likely to depend on the scanning angle. However, when avertically-variable resolution is given to a plurality of light-emittingelements in such a configuration, a plurality of light beams, which arevertically separated from each other, overlaps with each other afterreflected by the mirror, resulting in failure of a successfulmulti-layer detection in a vertical direction.

Further, the light-emitting system 10 preferably includes a light sourcethat includes a plurality of light-emitting elements to divide alight-emission range into a plurality of detection areas for detection.

This configuration that divides the light-emission range into areasusing a plurality of light-emitting elements employs fewerlight-receiving elements than cases where the light-emission range isdivided into area using a plurality of light-receiving elements.Accordingly, crosstalk in a received-light signal decreases in theconfiguration, thus increasing the accuracy of the detection.

In some embodiments, the object detector 100 may include a plurality offirst light detectors 43 to divide the light-emission range into aplurality of detection areas.

Preferably, a plurality of detection areas is divided into fewer groupsthan the number of detection areas. The voltage adjuster 200 preferablymakes the threshold voltage different from each other between groups.

Particularly in the scanning-object detector, the detection range mightbe divided into several hundred to several thousand detection areas.However, dividing the detection range into a small number of groupswhile using a small number of discrete values of threshold voltage canprevent an increase in size of circuit as well as effectively increasethe detection distance in areas having a lower light utilizationefficiency.

The voltage adjuster 200 includes a pulse-signal generating circuit 200a, a smoothing unit 200 b, and a duty controller 200 c. The pulse-signalgenerating circuit 200 a generates a pulse signal that is variable induty ratio. The smoothing unit 200 b smooths the generated pulse signal.The duty controller 200 c controls a duty ratio of the pulse signal.

The configuration of the smoothing unit 200 b is easily achieved byusing the RC low pass filter, and a control circuit may be used as thepulse-signal generating circuit 200 a that is capable of changing a dutyratio of the pulse signal. Accordingly, any voltage-variabledirect-current power source may not be used for controlling thethreshold voltage, which is advantageous in a reduction in cost andsize.

Note that, in some embodiments, a voltage-variable direct-current powersource may be used as the voltage adjuster 200.

From the second viewpoint (the detection sensitivity), the objectdetector 100 according to the present embodiment includes thelight-emitting system 10, the first light detector 43, and the firstbinarizing circuit 44. The first light detector 43 includes alight-receiving element that receives light emitted from thelight-emitting system 10 and reflected or scattered by an object. Thefirst binarizing circuit 44 (a signal detector) receives an outputsignal (a received-light signal) from the first light detector 43 todetect the output signal based on a threshold value. The first lightdetector 43 changes a degree of detection sensitivity between when thelight-emitting system 10 emits light to a part of a light-emission rangeof the light-emitting system 10 and when the light-emitting system 10emits light to another part of the light-emission range.

In this case, for example, the light-receiving sensitivity of thelight-receiving element or the amplification factor of the signalamplifier 60 b are made different from each other between detectionareas in the first light detector 43. Accordingly, the relation of thethreshold voltage and the noise level can approximate a desiredrelation. Thus, such a configuration can prevent a reduction indetection distance within the light-emission range.

The object detector 100 further includes the light receiving opticalsystem 30 to guide light reflected by an object to the first lightdetector 43. In the light receiving optical system 30, the lightutilization efficiency in emitting light to any edge of thelight-emission range is lower than in emitting light to the center ofthe light-emission range. In the object detector 100, the detectionsensitivity is preferably greater in emitting light to any edge of thelight-emission range than in emitting light to the center of thelight-emission range.

This configuration can achieve an object detector that is advantageousfor a wide-angle multi-layer detection. In non-scanning object detectorsas well, reducing the light utilization efficiency at any edge of thedetection range as compared to the light utilization efficiency at thecenter of the detection range can reduce the effective diameter of theoptical element, thus downsizing the object detector. Further, reducingthe detection sensitivity at any edge of the detection range as comparedto the detection sensitivity at the center of the detection range canprevent a reduction in detection distance due to the downsizing of theobject detector.

The light-emitting system 10 includes a light source, e.g., the LD 111,a scanning device, e.g., the rotating mirror 26 to scan thelight-emission range with light emitted from the light source. In thiscase, a detection sensitivity is preferably set according to an angle atwhich the scanning device scans the light-emission range with the light.

The light-emitting system 10 preferably employs an incident opticalsystem to cause the light emitted from the light source to enter thescanning device from the outside of the light-emission range.

In the configuration that makes a light beam enter the scanning devicefrom the outside of the light-emission range (scanning angle range),vignetting occurs at the edges of the light-emission range (any edge ofthe scanning angle range), and thereby the light utilization efficiencysignificantly varies with the scanning angle. Thus, such a configurationcan exhibit the advantageous effects of the present disclosure. Theabove-described configuration is particularly advantageous for caseswhere a plurality of light-emitting elements is vertically disposedhaving a vertically-variable resolution.

Further, the light-emitting system 10 may include a light source thatincludes a plurality of light-emitting elements to divide alight-emission range into a plurality of detection areas for detection.

This configuration that divides the light-emission range into areasusing a plurality of light-emitting elements employs fewerlight-receiving elements than cases where the light-emission range isdivided into area using a plurality of light-receiving elements.Accordingly, in such a configuration, crosstalk in a received-lightsignal decreases, thus increasing the accuracy of the detection.

In some embodiments, a plurality of first light detectors 43 may bedisposed to divide the light-emission range into a plurality ofdetection areas for detection. In this case, at least two(light-receiving elements) of the plurality of first light detectors 43may differ in light-receiving sensitivity.

In such a case, the degree of detection sensitivity can be determinedaccording to a detection area of the light-emission range by using onlythe light-receiving elements, so that no detection sensitivity adjusteris employed.

Preferably, a plurality of detection areas are divided into fewer groupsthan the number of detection areas. The degree of detection sensitivitypreferably differ between groups.

Particularly in the scanning-object detector, the detection range mightbe divided into several hundred to several thousand detection areas.However, dividing the detection range into a small number of groupswhile using a small number of discrete values of the threshold voltagecan prevent increasing the size of a circuit as well as effectivelyincrease the detection distance in areas having a lower lightutilization efficiency.

The first light detector 43 further includes a signal amplifier 60 b (asignal amplifying device) capable of changing an amplification factor toamplify a voltage signal according to an output electric current of thelight-receiving element. The object detector 100 may further include adetection sensitivity adjuster (a sensitivity adjuster) to adjust theamplification factor of the signal amplifier 60 b to change the degreeof detection sensitivity.

The light-receiving element of the first light detector 43 may furtherinclude a detection sensitivity adjuster (a sensitivity adjuster) tochange the level of the light-receiving sensitivity according to anapplied voltage to control an applied voltage, thus changing the degreeof detection sensitivity.

Preferably, the object detector 100 further includes a calculator thatincludes the time measuring device 45 and the measurement controller 46.The calculator calculates a distance to an object based on thelight-emitting timing of the light source and the light-receiving timingof the first light detector 43. This configuration can accuratelydetermine the distance to an object within the light-emission range.

According to the mobile apparatus 600 including the object detector 100and the mobile object 400 equipped with the object detector 100, areduction in detection distance within the projection range (effectivescan area) can be prevented, thus providing a mobile apparatus with anexcellent safety.

FIG. 13 is an illustration of a sensing device 1000 equipped with theobject detector 100. The sensing device 1000, which is mounted on amobile object 400, includes the object detector 100 and a monitoringcontroller 300 electrically connected to the object detector 100. Theobject detector 100 is mounted near a bumper or a rear-view mirror in avehicle (the mobile object 400). The monitoring controller 300, based onthe detection results of the object detector 100, estimates the size orshape of an object, and calculates the position and movement data of theobject, recognizing the type of the object. The monitoring controller300 ultimately judges presence of danger. The monitoring controller 300having made an affirmative judgment alerts an operator of the mobileobject 400 to the danger. Alternatively, the monitoring controller 300having an affirmative judgment issues an order to a steering controllerof the mobile object 400 to avoid the danger by steering, or issues anorder to the ECU to brake the mobile object 400. Note that the sensingdevice 1000 receives power supply from a vehicle battery, for example.The monitoring controller 300 may be implemented by, for example, acentral processing unit (CPU) or a control circuit.

Further, the monitoring controller 300 may be integrated with the objectdetector 100. Alternatively, in some embodiment, the monitoringcontroller 300 may be separate from the object detector 100. In someembodiments, the monitoring controller 300 may perform at least some ofthe control function of the ECU.

The sensing device 1000 according to the present embodiment includes theobject detector 100 and the monitoring controller 300. The monitoringcontroller 300, in response to the output of the object detector 100,obtain object data (at least one of the presence of an object, theposition of the object, the direction of movement of the object, and thespeed of movement of the object). With this configuration, the sensingdevice 1000 reliably and accurately obtains the object data.

The sensing device 1000 is mounted on the mobile object 400. Themonitoring controller 300, based on at least one of the position dataand the movement data of the object, judges the presence of danger,thereby providing effective data for avoiding danger to an operationalcontrol system and a speed control system of a mobile object 400, forexample.

Further, according to the mobile apparatus 600 including the mobileobject 400 and the sensing device 1000 mounted on the object detector100 or the mobile object 400, an excellent safety against impact can beprovided.

[Variation of Object Detector]

FIG. 14 is a block diagram of a schematic configuration of the objectdetector 100 according to a variation. The object detector 100 accordingto the present variation includes the LD 111, the projection opticalsystem 20, the LD drive device 12, the light receiving optical system30, the light-receiving element (PD or APD), the processing circuit(waveform processing circuit) 60, and the binarizing circuit 44. Thatis, in the present variation, the object detector 100 may not includethe time measuring device 45, the measurement controller 46, the objectrecognizer 47, and the synchronous system 50. Note that the synchronoussystem 50 is used in a scanning object detector, but not used in anon-scanning object detector.

The object detector 100 according to the present variation also has thesame configuration to determine the threshold voltage or the detectionsensitivity according to a detection area, as in the object detector 100according to the above-described embodiment. That is, the objectdetector 100 according to the present variation includes, for example, avoltage adjuster 200, a detection adjuster, or a plurality oflight-receiving elements having different degrees of light-receptivesensitivity.

According to the above-described embodiment, an LD is used as a lightsource. However, the present disclosure is not limited to thisconfiguration. In some embodiment, other types of light emittingelements, such as vertical-cavity surface-emitting lasers (VCSELs),organic electroluminescence (EL) elements and LEDs, may be employed as alight source.

The processing circuit 60 may include only a current-voltage converter60 a. Alternatively, the processing circuit 60 may include acurrent-voltage converter 60 a and a high-pass filter 60 c. In otherwords, the processing circuit 60 may not include a signal amplifier.

The projection optical system 20 may not include the coupling lens 22.Alternatively, the projection optical system 20 may include another typeof lens.

The projection optical system 20 and the light receiving optical system30 may not include the reflection mirror 24. That is, light emitted fromthe LD 111 may enter the rotating mirror 26 without the folded opticalpath.

Further, the projection optical system 20 may include any other opticalelement, such as a condenser mirror, instead of an image-forming lens.

Further, a deflector may be any other mirror, such as a polygon mirror(rotating polygon mirror), a galvano mirror, or a micro electromechanical system (MEMS) mirror, instead of the rotating mirror 26.

The synchronous system 50 may not include the synchronization lens 52.Alternatively, the synchronous system 50 may include any other opticalelement, such as a condenser mirror.

According to the above-described embodiment, an automobile is taken asan example for the mobile object 400 equipped with the object detector100. Examples of the mobile object 400 may include a vehicle other thanan automobile, an airplane, an unmanned aerial vehicle, a vessel, and arobot.

Further, specific numerical values and shapes taken for the abovedescription are illustrative only, and can be modified as appropriatewithout exceeding beyond the scope of the present disclosure.

As is apparent from the above-description, the object detector 100, thesensing device 1000, and the mobile apparatus 600 allows for thetechnology that measures the distance to an object, utilizing the Timeof Flight (TOF) method or the technology used in the TOF. Such atechnology is widely used in the industries of the motion-capturetechnology, the range instruments, and the three-dimensional shapemeasurement technology, in addition to the sensing in a mobile object400. Therefore, the object detector 100 and the sensing device 1000according to the present disclosure may not be mounted on a mobileobject 400.

Numerous additional modifications and variations are possible in lightof the above teachings. It is therefore to be understood that, withinthe scope of the above teachings, the present disclosure may bepracticed otherwise than as specifically described herein. With someembodiments having thus been described, it will be obvious that the samemay be varied in many ways. Such variations are not to be regarded as adeparture from the scope of the present disclosure and appended claims,and all such modifications are intended to be included within the scopeof the present disclosure and appended claims.

What is claimed is:
 1. An object detector comprising: a light-emittingsystem; a light detector including a light-receiving element to receivelight emitted from the light-emitting system and reflected by theobject, and output a signal; and a signal detector to detect the signaloutput from the light detector based on a threshold value of voltage,wherein a degree of sensitivity of the light detector differs betweenwhen the light-emitting system emits light to a part of a light-emissionrange of the light-emitting system and when the light-emitting systememits light to another part of the light-emission range other than thepart of the light-emission range, wherein a ratio of the threshold valueof voltage relative to a standard deviation of noise is constant betweenwhen the light-emitting system emits light to the part of alight-emission range of the light-emitting system and when thelight-emitting system emits light to the another part of thelight-emission range other than the part of the light-emission range. 2.The object detector according to claim 1, further comprising a lightreceiving optical system to guide the light reflected by the object tothe light detector, wherein light utilization efficiency of the lightreceiving optical system is lower in an emission of light to an edge ofthe light-emission range than in the emission of light to a center ofthe light-emission range, and the degree of sensitivity of the lightdetector is greater in the emission of light to the edge of thelight-emission range than in the emission of light to the center of thelight-emission range.
 3. The object detector according to claim 1,wherein the light-emitting system includes a light source and a scanningdevice to scan the light-emission range with light emitted from thelight source, and wherein the degree of sensitivity of the lightdetector changes with a scan angle of the scanning device.
 4. The objectdetector according to claim 3, wherein the light-emitting systemincludes an incident optical system to cause the light emitted from thelight source to enter the scanning device from an outside of thelight-emission range.
 5. The object detector according to claim 1,wherein the light-emitting system includes a light source including aplurality of light-emitting elements to divide the light-emission rangeinto a plurality of detection areas for detection.
 6. The objectdetector according to claim 5, wherein the plurality of detection areasis divided into a plurality of groups fewer than a number of theplurality of detection areas, and wherein the degree of sensitivity ofthe light detector differs between the plurality of groups.
 7. Theobject detector according to claim 1, wherein the light detector is aplurality of light detectors to divide the light-emission range into aplurality of detection areas for detection, and wherein at least two ofthe plurality of light detectors change in light-receiving sensitivityfor each light-receiving element.
 8. The object detector according toclaim 1, wherein the light detector further includes a signal amplifier,which is variable in an amplification factor, to amplify a voltagesignal according to an electric current output from the light-receivingelement, and wherein the degree of sensitivity of the light detectorchanges with the amplification factor of the signal amplifier.
 9. Theobject detector according to claim 1, wherein the light-receivingelement changes in light-receiving sensitivity according to a voltageapplied to the light detector.
 10. A sensing device comprising: theobject detector according to claim 1; and a monitoring controller thatdetermines at least one of a presence or an absence of the object, adirection of movement of the object, and a moving speed of the object,based on an output of the object detector.
 11. A mobile objectcomprising: a mobile object; and the sensing device according to claim10 mounted on the mobile object.
 12. A mobile apparatus comprising: amobile object; and the object detector according to claim 1 mounted onthe mobile object.
 13. The object detector according to claim 1,wherein: a degree of the noise of the light detector is proportional toa square root of the light that has been received by the light detectionmeans.
 14. An object detector comprising: a light-emitting system; lightdetection means including a light-receiving element to receive lightemitted from the light-emitting system and reflected by the object, andoutput a signal; and a signal detector to detect the signal output fromthe light detection means based on a threshold value of voltage, whereina degree of sensitivity of the light detection means differs betweenwhen the light-emitting system emits light to a part of a light-emissionrange of the light-emitting system and when the light-emitting systememits light to another part of the light-emission range other than thepart of the light-emission range, wherein a ratio of the threshold valueof voltage relative to a standard deviation of noise is constant betweenwhen the light-emitting system emits light to the part of alight-emission range of the light-emitting system and when thelight-emitting system emits light to the another part of thelight-emission range other than the part of the light-emission range.15. The object detector according to claim 14, further comprising alight receiving optical system to guide the light reflected by theobject to the light detection means, wherein light utilizationefficiency of the light receiving optical system is lower in an emissionof light to an edge of the light-emission range than in the emission oflight to a center of the light-emission range, and the degree ofsensitivity of the light detection means is greater in the emission oflight to the edge of the light-emission range than in the emission oflight to the center of the light-emission range.
 16. The object detectoraccording to claim 14, wherein the light-emitting system includes alight source and a scanning device to scan the light-emission range withlight emitted from the light source, and wherein the degree ofsensitivity of the light detection means changes with a scan angle ofthe scanning device.
 17. The object detector according to claim 16,wherein the light-emitting system includes an incident optical system tocause the light emitted from the light source to enter the scanningdevice from an outside of the light-emission range.
 18. The objectdetector according to claim 14, wherein the light-emitting systemincludes a light source including a plurality of light-emitting elementsto divide the light-emission range into a plurality of detection areasfor detection.
 19. The object detector according to claim 14, wherein: adegree of the noise of the light detector is proportional to a squareroot of the light that has been received by the light detector.